The present invention relates in general to the production of mono-olefins by the oxidative dehydrogenation of gaseous paraffinic hydrocarbons having two or more carbon atoms and in particular to the production of mono-olefins by autothermal cracking, especially of ethane, propane, and butanes.
A known commercial route to the production of olefins is via steam cracking of paraffinic hydrocarbons. Steam cracking involves pyrolysis of the hydrocarbons. Olefins can also be prepared by cracking a paraffinic feed wherein the heat required for pyrolysis is provided by the partial combustion of the feedstock and not by conventional tubular fired heaters as in the steam cracking. This process can be described as xe2x80x9cautothermal crackingxe2x80x9d and will be described as such hereinafter. Autothermal cracking may be accomplished in the absence of a catalyst, as described in for example, an article entitled xe2x80x9cAutothermal Cracking for Ethylene Productionxe2x80x9d by R. M. Deanesly in Petrol. Refiner, 29 (September, 1950), 217 and GB-A-794,157, or in the presence of a catalyst capable of supporting combustion beyond the normal fuel rich limit of flammability as described in, for example, EP-A-0164864; EP-A-0178853; EP-B1-0332289; EP-B1-0529793; and EP-A-0709446.
EP-A-0164864 and EP-A-0178853 describe the production of olefins together with carbon monoxide and hydrogen from gaseous paraffinic hydrocarbons, including ethane, by partial oxidation in spouted or fluid bed reactors. EP-A-0178853, for example, discloses a process for the preparation of synthesis gas (carbon monoxide and hydrogen from a hydrocarbon feed wherein the saturated hydrocarbon and oxygen-containing gas are introduced in a ratio of 1.1 to 5 times the stoichiometric ratio of hydrocarbon to oxygen for complete combustion. This process does not require a catalyst for its operation.
EP-B1-0332289 provides a process for the production of a mono-olefin from a gaseous paraffinic hydrocarbon having at least two carbon atoms or a mixture thereof by partially combusting a mixture of the hydrocarbon(s) and a molecular oxygen-containing gas in a composition of from 5 to 9 times the stoichiometric ratio of hydrocarbon to molecular oxygen-containing gas for complete combustion to carbon dioxide water, in contact with a catalyst capable of supporting combustion beyond the normal fuel rich limit of flammability.
EP-B1-0529793 discloses a process for the production of mono-olefins from a paraffin-containing hydrocarbon feed having at least two carbon atoms, the process comprising
(A) a first step of partially combusting a mixture of the hydrocarbon feed and a molecular oxygen-containing gas in contact with a catalyst capable of supporting combustion beyond the normal fuel rich limits of flammability, said first step carried out under a total pressure of greater than 5 bar absolute and at a temperature of greater than 650xc2x0 C., and
(B) a second step of cooling the mono-olefinic products to 600xc2x0 C. or less within less than 50 milliseconds of formation.
Finally, EP-A-0709446 discloses a process for the conversion of a liquid paraffin-containing hydrocarbon which comprises the step of:
(a) partially combusting a mixture of the liquid hydrocarbon and a molecular oxygen-containing gas in a reaction chamber with a catalyst capable of supporting combustion beyond the normal fuel rich limit of flammability, the mixture having a stoichiometric ratio of hydrocarbon to oxygen of greater than the stoichiometric ratio required for complete combustion to carbon dioxide and water, to produce a product stream and a carbon deposit in the reaction chamber;
(b) periodically replacing the liquid hydrocarbon and molecular oxygen-containing gas mixture in step (a) with a fuel-rich carbon-containing stream for a period of time sufficient to effect substantial removal of the carbon deposit from the reaction chamber.
In addition to the valuable olefinic product(s) the autothermal cracking reaction produces both carbon monoxide and hydrogen, which in admixture is hereinafter to be referred to as xe2x80x9csynthesis gasxe2x80x9d. The resulting synthesis gas from autothermal cracking is usually used as a fuel. This is a problem because it represents a waste of a valuable resource and therefore imposes an economic penalty on the process. A solution to this problem is to recover the synthesis gas and convert it to higher value products.
Accordingly, the present invention provides a process for the production of a mono-olefin from a feedstock comprising a paraffinic hydrocarbon which process comprises the steps of:
(a) feeding the feedstock and a molecular oxygen-containing gas to an autothermal cracker, where they are reacted by oxidative dehydrogenation to form a product comprising one or more mono-olefin(s) and synthesis gas,
(b) separating the product from step (a) into a synthesis gas-containing stream and one or more olefins and recovering the one or more olefin(s),
(c) contacting synthesis gas-containing stream separated in step (b) with either:xe2x80x94
(i) a catalyst for the conversion of synthesis gas to methanol under conditions whereby synthesis gas is converted to methanol, and optionally thereafter or simultaneously contacting the methanol so-formed with a catalyst for the dehydration of methanol to ethylene, under conditions whereby methanol is converted to ethylene and recovering the ethylene, or optionally methanol or both; or
(ii) a catalyst for the water gas shift reaction under conditions whereby carbon monoxide in the synthesis gas is converted by reaction with water to hydrogen and carbon dioxide and thereafter recovering hydrogen; or
(iii) a catalyst for the conversion of synthesis gas to hydrocarbons under conditions whereby synthesis gas is converted to a hydrocarbon product and thereafter recovering at least a part of the hydrocarbon product.
The term xe2x80x9coxidative dehydrogenationxe2x80x9d is not intended to be limiting in any way and broadly describes the chemistry involved. It is believed that both partial oxidation and cracking occur in the reaction.
The feedstock may be oxidatively dehydrogenated in the autothermal cracker in the presence of absence of a catalyst capable of supporting combustion beyond the normal fuel rich limit of flammability, preferably in the presence of such a catalyst. The principal role of the catalyst is to stabilise partial combustion of the gaseous mixture which may not otherwise be flammable. Suitably the catalyst is a supported platinum group metal. Preferably, the metal is either platinum or palladium, or a mixture thereof. Although a wide range of support material are available, it is preferred to use alumina as the support. The support material may be in the form of spheres, other granular shapes or ceramic foams. Preferably, the foam is a monolith which is a continuous multichannel ceramic structure, frequently of a honeycomb appearance. A preferred support for the catalytically active metals is a gamma alumina. The support is loaded with a mixture of platinum and palladium by conventional methods well known to those skilled in the art. The resulting compound is then heat treated to 1200xc2x0 C. before use. Catalyst promoters may also be loaded onto the support. Suitable promoters include copper and tin.
The catalyst may be used as a fixed bed or as a solids recirculating bed, for example a fluid or spouted bed. Use of the catalyst in a fixed bed can avoid the problem of attrition, which is mainly associated with moving bed operations. Moreover, the use of a fixed bed also facilitates rapid quenching of the products as compared with the use of a fluid bed.
In the process for the production of a mono-olefin from a feedstock comprising a gaseous paraffinic hydrocarbon the paraffinic hydrocarbon may suitably be ethane, propane or butane. The paraffinic hydrocarbon may be substantially pure or may be in admixture with other hydrocarbons and optionally other materials, for example methane, nitrogen, carbon monoxide, carbon dioxide, steam or hydrogen. A paraffinic hydrocarbon-containing fraction such as naphtha, gas oil, vacuum gas oil, or mixtures thereof, may be employed. A suitable feedstock is a mixture of gaseous paraffinic hydrocarbons, principally comprising ethane, resulting from the separation of methane from natural gas. A preferred feedstock is a paraffinic hydrocarbon principally comprising ethane which provides a product principally comprising ethylene as the mono-olefin.
As the molecular oxygen-containing gas there may suitably be used either oxygen or air. It is preferred to use oxygen, optionally diluted with an inert gas, for example nitrogen. It is preferred to pre-mix the oxygen-containing gas and the paraffinic hydrocarbons feedstock prior to contact with the catalyst, when present. In the presence of a catalyst the composition of the gaseous paraffinic hydrocarbon/molecular oxygen-containing gas mixture is suitably from 5 to 13.5 times the stoichiometric ratio of hydrocarbon to oxygen-containing gas for complete combustion to carbon dioxide and water. The preferred composition is from 5 to 9 times the stoichiometric ratio of hydrocarbon to oxygen-containing gas.
The autothermal cracker may suitably be operated at a temperature greater than 500xc2x0 C., for example greater than 650xc2x0 C., typically greater than 750xc2x0 C., and preferably greater than 800xc2x0 C. The upper temperature limit may suitably be up to 1200xc2x0 C., for example up to 1100xc2x0 C., preferably up to 1000xc2x0 C.
It is preferred, although not essential, to preheat the feedstock and the oxygen-containing gas to suitably 200 to 500xc2x0 C., preferably 200-300xc2x0 C.
The autothermal cracker may be operated at atmospheric or elevated pressure. Pressures of 1 to 30 bara may be suitable, preferably a pressure of 1.1 bara to 5 bara, for example, 1.8 bara is employed. However, a total pressure of greater than 5 bar absolute can also be suitably employed.
Preferably, the gaseous feedstock and the molecular oxygen-containing gas are fed to the autothermal cracker in admixture under a Gas Hourly Space Velocity (GHSV) of greater than 80,000 hrxe2x88x921 in order to minimise the formation of carbon monoxide and carbon dioxide. Preferably, the GHSV exceeds 200,000 hr31 1, especially greater than 1,000,000 hrxe2x88x921. For the purposes of the present invention GHSV is defined as:xe2x80x94
vol. of total feed at NTP/Time/(vol. of catalyst bed).
For further details of preferred methods of operation reference may be made to the aforesaid EP-B1-0332289; EP-B1-0529793; and EP-A-0709446.
In addition to mono-olefins and synthesis gas, small amounts of acetylenes, aromatics and carbon dioxide are generally co-produced.
In step (b) of the process of the invention the product from step (a) of the process is separated into synthesis gas-containing stream and one or more mono-olefin(s) and the one or more mono-olefin(s) are recovered. Means for separating synthesis gas from mono-olefins are well-known in the art.
In step (c) of the process of the invention gas-containing stream separated in step (b) is contacted in alternative (i) with a catalyst for the conversion of synthesis gas to methanol under conditions whereby synthesis gas is converted to methanol and optionally thereafter, or simultaneously, the methanol so-formed is contacted with a catalyst for the dehydration of methanol to ethylene under conditions whereby methanol is converted to ethylene and the ethylene, or optionally the methanol, or both, is recovered.
In alternative (ii) of step (c) of the process synthesis gas-containing stream separated in step (b) is contacted with a catalyst for the water gas shift reaction whereby carbon monoxide in the synthesis gas is converted by reaction with water to hydrogen and carbon dioxide and thereafter hydrogen is recovered. The water gas shift reaction may be represented as:xe2x80x94
CO+H2O=CO2+H2 xe2x80x83xe2x80x83(I) 
The water gas shift reaction is well-known in the art. It is generally operated in the presence of a catalyst. Typically an iron oxide catalyst may be employed, although other catalysts know in the art may be employed. Temperatures typically in the range from 350 to 500xc2x0 C. may suitably be used. Conditions known in the art for driving the equilibrium towards the formation of hydrogen are preferably employed.
The water reactant may be some or all of the water produced as a by-product in the oxidative dehydrogenation of the paraffinic hydrocarbon. Additionally further water may be added if desired.
In alternative (iii) of step (c) of the process of the invention synthesis gas-containing stream separated in step (b) is contacted with a catalyst for the conversion of synthesis gas to hydrocarbons under conditions whereby synthesis gas is converted to a hydrocarbon product, and thereafter at least a part of the hydrocarbon product is recovered.
The conversion of synthesis gas into hydrocarbons has been known for many years, it being generally known as the Fischer-Tropsch (FT) process. The process produces hydrocarbons from C1 upwards, but principally in the C5-C60 range. In recent years attenuation has been directed to the FT process as one of the more attractive direct and environmentally acceptable routes to high quality transportation fuels from alternative energy sources such as coal and natural gas via intermediate formation of synthesis gas. Thus, there may be recovered from the FT product a desirable hydrocarbon fraction boiling in the diesel range. There may also be recovered a less valuable fraction of lower boiling point, which fraction is generally referred to as naphtha. Not all of this fraction is industrially utiliseable. In a preferred embodiment the recovered naphtha fraction is recycled at least in part as feedstock to the autothermal cracker in the process of the present invention. This leads to the advantage that the FT process and the process for the production of a mono-olefin by the oxidative dehydrogenation of a gaseous paraffinic hydrocarbon complement each other in the respect that the synthesis gas by-product of the oxidative dehydrogenation is useful as feed to the FT process and the lesser value naphtha product of the FT process is useful as a feedstock in the oxidative dehydrogenation. Moreover, a significant additional advantage is that the high pressure xe2x80x9cfuel gasxe2x80x9d stream comprising methane, carbon monoxide and hydrogen obtained from the conventional autothermal cracking separation via a demethaniser in step (II) may be used directly as feedstock to the FT reactor in step (III) wherein the carbon monoxide/hydrogen component is consumed leaving methane and residual carbon monoxide as a more conventional fuel gas. Thus, the FT reactor functions as a reactor/separator adding value to the carbon monoxide/hydrogen and eliminating a separation stage.
Accordingly, in a preferred embodiment the present invention provides an integrated process for the production of a mono-olefin and a hydrocarbon fraction boiling in the diesel range which process comprises the steps of:
(I) feeding a gaseous paraffinic hydrocarbon-containing feedstock and a molecular oxygen-containing gas to an autothermal cracker wherein they are reacted in the presence or absence of a catalyst capable of supporting combustion beyond the normal fuel rich limit of flammability under conditions whereby the feedstock is oxidatively dehydrogenated to a product comprising one or more mono-olefin(s) and synthesis gas,
(II) separating the product from step (I) into synthesis gas and one or more mono-olefin(s) and recovering the one or more olefin(s),
(III) feeding synthesis gas separated in step (II), optionally together with additional synthesis gas, to an FT reactor containing an FT catalyst wherein the synthesis gas is reacted under FT conditions to produce an FT product comprising naphtha and hydrocarbons boiling in the diesel range,
(IV) separating the FT product from step (III) into a naphtha fraction and a diesel range hydrocarbon fraction and recovering the diesel range hydrocarbon fraction, and
(V) preferably recycling the naphtha fraction recovered in step (IV) as feed to the autothermal cracker of step (I).
Steps (I) and (II) of the preferred embodiment have been described hereinbefore.
As regards step (III) the catalyst may suitably comprise at least one metal selected from cobalt, nickel, iron, molybdenum, tungsten, thorium, ruthenium, rhenium, and platinum. Of the aforesaid metal cobalt, nickel and iron are preferred. Generally, the metals may be used in combination with a support material. Suitable support materials include alumina, silica and carbon, and mixtures of two or more thereof. The use of cobalt, for example, as a catalytically active metal in combination with a support is well-known from, for example EP-A-127220; EP-A-142887; GB-A-2146350; GB-A-2130113; EP-A-0209980; EP-A-0261870 and GB-A-2125062. Of these EP-A-127220, for example, discloses the use of a catalyst comprising (i) 3-60 pbw cobalt, (ii) 0.1-100 pbw zirconium, titanium, ruthenium or chromium, per 100 pbw silica, alumina or silica-alumina, (iii) the catalyst having been prepared by kneading and/or impregnation. EP-A-0209980 describes the use in the conversion of synthesis gas to hydrocarbons of a catalyst having a composition represented by the formula:
COa.Ab.Lac.CeOx 
wherein
A is an alkali metal
a is greater than zero and up to 25% w/w,
b is in the range from zero to 5% w/w,
c is in the range from zero to 15% w/w,
x is a number such that the valence requirements of the other elements for oxygen is satisfied, and the remainder of the composition, subject to the requirement for x, is cerium.
EP-A-0261870 discloses a composition for use after reductive activation as a catalyst in the conversion of synthesis gas to hydrocarbons, which composition comprises as essential components (i) cobalt either as the element metal, the oxide or a compound thermally decomposable to the element metal and/or oxide and (ii) zinc in the form of the oxide or a compound thermally decomposable to the oxide.
Any of the aforesaid catalysts may be used in the process of the present invention and for further details of the catalysts their preparation and use reference may be made to the publications.
FT conditions are suitably a temperature in the range from 160 to 350xc2x0 C., preferably from 180 to 275xc2x0 C., and a pressure in the range from 0 to 100 bar, preferably from 5 to 50 bar. The GHSV for continuous operation may suitably be in the range from 100 to 2500 hxe2x88x921.
The process of step (III) may be carried out batchwise or continuously, preferably continuously, in a fixed bed, fluidised bed or slurry phase reactor.
The additional synthesis gas is suitably obtained by the partial oxidation of a carbonaceous substance, e.g. coal or by catalytic partial oxidation of methane, or liquified petroleum gases. Preferably, the synthesis gas is obtained by the catalytic steam reforming of methane, either as such or in the form of natural gas. Steam reforming and catalysts therefor are well-known in the art. Typically, a supported nickel catalyst is employed. The carbon monoxide to hydrogen ratio of the synthesis gas may suitably be in the range from 2:1 to 1:6, preferably from 2:1 to 1:2. Whilst the ratio of carbon monoxide to hydrogen in the synthesis gas produced by the aforesaid processes may differ from these ranges, it may be altered appropriately by the addition of either component, or may be adjusted by the shift reaction as mentioned hereinbefore.
In step (IV) of the preferred embodiment the FT product from step (III) is separated into a naphtha fraction and a diesel range hydrocarbon fraction. This separation may be accomplished by any of the means known in the art. Generally, the fractions may be separated by distillation. Preferably, the diesel range hydrocarbon produced in step (IV) is highly paraffinic, with good combustion characteristics. Preferably also, the diesel range hydrocarbon substantially no metal, nitrogen, sulfur or aromatic impurities. If such impurities are present however, their concentrations are kept to a minimum. For example, the sulpher content of diesel fraction may be less than 0.01 wt %, and more preferably less than 0.001% weight. The diesel range hydrocarbon may have a density of 0.7 to 0.9 g/ml., preferably, 0.8 g/ml. Its pour point may be between xe2x88x9235 to xe2x88x9245xc2x0 C., preferably about xe2x88x9238 to xe2x88x9240xc2x0 C.
In step (V) the naptha fraction recovered in step (IV) is preferably recycled as feed to the autothermal cracker of step (I). Alternatively some, or all, the naphtha fraction may be recovered, and used elsewhere.